Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395, 5,657,607, and 5,902,652, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1-2 illustrate a conventional vacuum IG unit (vacuum IG unit or VIG unit). Vacuum IG unit 1 includes two spaced apart glass substrates 2 and 3, which enclose an evacuated or low pressure space 6 there between. Glass sheets/substrates 2 and 3 are interconnected by peripheral or edge seal of fused solder glass 4 and an array of support pillars or spacers 5.
Pump out tube 8 is hermetically sealed by solder glass 9 to an aperture or hole 10 which passes from an interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. A vacuum is attached to pump out tube 8 so that the interior cavity between substrates 2 and 3 can be evacuated to create a low pressure area or space 6. After evacuation, tube 8 is melted to seal the vacuum. Recess 11 retains sealed tube 8. Optionally, a chemical getter 12 may be included within recess 13.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows. Glass frit in a solution (ultimately to form solder glass edge seal 4) is initially deposited around the periphery of substrate 2. The other substrate 3 is brought down over top of substrate 2 so as to sandwich spacers 5 and the glass frit/solution there between. The entire assembly including sheets 2, 3, the spacers, and the seal material is then heated to a temperature of approximately 500° C., at which point the glass frit melts, wets the surfaces of the glass sheets 2, 3, and ultimately forms hermetic peripheral or edge seal 4. This approximately 500° C. temperature is maintained for from about one to eight hours. After formation of the peripheral/edge seal 4 and the seal around tube 8, the assembly is cooled to room temperature. It is noted that column 2 of U.S. Pat. No. 5,664,395 states that a conventional vacuum IG processing temperature is approximately 500° C. for one hour. Inventors Lenzen, Turner and Collins of the '395 patent have stated that “the edge seal process is currently quite slow: typically the temperature of the sample is increased at 200° C. per hour, and held for one hour at a constant value ranging from 430° C. and 530° C. depending on the solder glass composition.” After formation of edge seal 4, a vacuum is drawn via the tube to form low pressure space 6.
The composition of conventional edge seals are known in the art. See, for example, U.S. Pat. Nos. 3,837,866; 4,256,495; 4;743,302; 5,051,381; 5,188,990; 5,336,644; 5,534,469; 7,425,518, and U.S. Publication No. 2005/0233885, the disclosures of which are all hereby incorporated herein by reference.
In certain instances, the aforesaid high temperatures and long heating times of the entire assembly utilized in the formulation of edge seal 4 are undesirable. This is especially the case when it is desired to use a heat strengthened or tempered glass as substrate(s) 2, 3 in the vacuum IG unit. As shown in FIGS. 3-4, tempered glass loses temper strength upon exposure to high temperatures as a function of heating time. Moreover, such high processing temperatures may adversely affect certain low-E coating(s) that may be applied to one or both of the glass substrates in certain instances.
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The x-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the y-axis is indicative of the percentage of original temper strength remaining after heat exposure. FIG. 4 is a graph similar to FIG. 3, except that the x-axis in FIG. 4 extends from zero to one hour exponentially.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (° F.). The different curves/lines are 400° F. (across the top of the FIG. 3 graph), 500° F., 600° F., 700° F., 800° F., 900° F., and 950° F. (the bottom curve of the FIG. 3 graph). A temperature of 900° F. is equivalent to approximately 482° C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal 4 in FIGS. 1-2. Thus, attention is drawn to the 900° F. curve in FIG. 3, labeled by reference number 18. As shown, only 20% of the original temper strength remains after one hour at this temperature (900° F. or 482° C.). Such a significant loss (i.e., 80% loss) of temper strength may be undesirable.
Further, when the temperature the sheet is exposed to is reduced to 800° F., about 428° C., the amount of strength remaining is about 70%. Finally, a reduction in temperature to about 600° F., about 315° C., results in about 95% of the original temper strength of the sheet remaining. Alternatively, or in addition, a reduced time period of exposure to high temperatures may decrease the temper strength loss. For example, 10 minutes of being exposed to approximately 900° F. may result a temper strength that is 60% to 70% of the original value. As will be appreciated, it may be desirable to reduce any temper strength losses as a result of exposing a tempered sheet of glass to high temperatures.
As noted above, the creation of VIG units includes the creation of a hermetic seal that can withstand the pressure applied from the vacuum created on inside of the unit. As also discussed above, the creation of the seal may conventionally involve temperatures of at or above 500° C. for periods of around one hour. These temperatures are required in order to obtain a high enough temperature for a conventional frit material to melt and form a seal for a VIG unit. As shown above, such a temperature can result in a strength reduction (often times a dramatic strength reduction) for VIG units using tempered glass.
The above temperatures are traditionally achieved through the use of a convection heating process (e.g., an ordinary oven). Such a heating process may be problematic for sealing a frit material between two substrates of glass. For example, the movement of air within a chamber via the convention process may affect the surface temperature of the glass substrates and may adversely affect the sealing process. It will be appreciated that temperature variations in the glass substrate may cause bending, warping, etc. These side effects may then prevent a frit material from forming a sufficient seal on a glass substrate (e.g., because the glass is not flat). In a convection oven the air temperature may be kept within a couple degrees throughout the oven. However, the temperature of a glass substrate may vary more than 10 degrees depending on the placement of a particular portion of the glass within the oven. Further, such temperature variations (and the associated problems) may be more pronounced as the temperature within the oven increases.
One conventional solution that may avoid the above problems is to use an epoxy to seal the substrates together. However, in the case of VIG units, epoxy compositions may be insufficient to hold a seal on a vacuum. Furthermore, epoxies may be susceptible to environmental factors that may further reduce their effectiveness when applied to VIG units.
Another conventional solution is to use a frit solution that contains lead. As is known, lead has a relatively low melting point. Accordingly, temperatures for sealing the VIG units may not need to be as high for other frit materials, and thus the tempering strength of tempered glass substrates may not be reduced by the same amount required for other frit based materials.
Typical lead based frits may contain between about 70% and 80% lead assay by weight. Such frits may have a sealing temperature (e.g., the temperature where the frit melts and bonds to the substrate) between about 400° C. and 500° C.
While lead based frits may resolve certain issues, the usage of lead in the frit may create new problems. Specifically, there may be health consequences as a result of products containing lead. Additionally, certain countries (e.g., in the European Union) may impose strict requirements on the amount of lead that can be contained in a given product. Indeed, some countries (or customers) may require products that are completely lead-free.
Thus, it will be appreciated that non-lead based frits are continuously sought after. Additionally, techniques for creating glass articles with non-lead based frits (e.g., seals) are continuously sought after. The frit material may be designed to allow for reduced temperature sealing such that annealed or tempered glass can be sealed without a substantial detrimental impact on the properties of the glass. Further, as discussed above, temperature variations across the surface of a glass substrate during a sealing processing may adversely affect the quality of the seal that is created. Accordingly, techniques for improving the sealing process for creating a seal or a VIG unit with a seal are continuously sought after.
A frit material according to certain example embodiments may have one or more of the following features and/or advantages:    a. A relatively low melting temperature versus other frits used for VIG units    b. Good glass/frit wetting and adhesion.    c. Sufficient melt flow to have some process tolerance for glass bow and frit height processing variation.    d. Frit melting tolerance for temperature range. Seal forms a reduced amount of bubbles and maintains adequate seal strength.    e. Coefficient of thermal expansion (CTE) match with a range to seal to glass.    f. Seal formed by frit is hermetic.    g. IR absorption properties are high or additives added for maximum use of near IR in an oven.    h. Low frit crystallization or crystallization at higher than sealing temperature to allow frit at sealing temperature to be fluid for wetting and flow ability.    i. Relatively fast bonding time to glass substrates.    j. A mechanical bond strength that is enough to with stand thermal stresses and/or vacuum induced stresses.
In certain example embodiments, a process of forming an edge seal or a VIG unit with an edge seal may include applying IR energy to the frit material. The IR energy may facilitate the melting and/or sealing of the frit material to a glass substrate.
In certain example embodiments a frit material is provided. The frit material includes a composition that includes bismuth oxide, zinc oxide, boric oxide, aluminum oxide, and magnesium oxide in amounts sufficient to absorb at least 80% of infrared (IR) energy having a wavelength of 1100-2100 nm.
In certain example embodiments a frit material is provided. A composition that includes bismuth oxide, zinc oxide, boric oxide, aluminum oxide, and magnesium oxide is included in the frit material. The frit material substantially melts through when the frit is held at a temperature of no greater than 525° C. for no more than about 3 minutes.
In certain example embodiments a method of making an edge seal for a VIG unit is provided. IR energy is applied from at least one IR emitter operating at a first voltage for a first predetermined period of time to a frit material. The operating voltage of the at least one IR emitter is reduced from the first voltage to a second voltage for a second predetermined period of time so as to reduce the IR energy impinging upon the frit material. The operating voltage of the at least one IR emitter is increased from the second voltage to a third voltage for a third predetermined period of time so as increase the IR energy impinging upon the frit material. The frit material is cooled or allowed to cool over a fourth predetermined period of time.
In certain example embodiments, a method of making a VIG unit is provided. A VIG subassembly is exposed to a first base temperature, the VIG subassembly includes first and second substantially parallel spaced apart glass substrates and a frit material is provided around a peripheral edge between the first and second substrates. IR energy is applied to the VIG subassembly so as to increase a temperature of the frit material to melting temperature range, wherein the melting temperature range is no greater than 525° C. The IR energy is maintained energy at the melting temperature range for no more than 5 minutes. A temperature of the at least two glass substrates does not exceed about 475° C. when the IR energy is maintained at the melting temperature range and a temperature difference across the surface of the first and second substrates does not exceed about +/−5° C. until the frit material hardens after reaching the melting temperature.
In certain example embodiments a method of making VIG unit is provided. First and second substantially parallel spaced apart glass substrates are provided with a frit material being provided at a peripheral edge thereof. IR energy is applied from at least one IR emitter operating at a first voltage for a first predetermined period of time to a frit material. The operating voltage of the at least one IR emitter is reduced from the first voltage to a second voltage for a second predetermined period of time so as to reduce the IR energy impinging upon the frit material. The operating voltage of the at least one IR emitter is increased from the second voltage to a third voltage for a third predetermined period of time so as increase the IR energy impinging upon the frit material. The frit material is cooled or allowed to cool over a fourth predetermined period of time.
The features, aspects, advantages, and example embodiments described herein may be combined in any suitable combination or sub-combination to realize yet further embodiments.